The present invention relates generally to integrated circuits, and, more particularly, to a level shifter circuit.
Level shifters are used in integrated circuits to shift voltage levels of signals that cross voltage domains that operate at different voltage levels. Phase-locked loop (PLL) circuits commonly use level shifters to shift the voltage level of an oscillating signal generated by a voltage-controlled oscillator (VCO) of the PLL. The frequency of the oscillating signal is controlled by an input control voltage. The VCO includes a voltage-to-current converter and a current-controlled oscillator (CCO). The voltage-to-current converter converts the input control voltage into a current input, and the CCO modulates the frequency of the oscillating signal based on the current input.
A ring oscillator is the most commonly used CCO and includes an odd number of cascade-connected inverters, for example, three cascade-connected inverters. Such a ring oscillator generates three oscillating signals that have a predetermined phase difference between them. Each oscillating signal oscillates from zero (i.e., ground voltage level) to a voltage level that is equal to a ring voltage of the CCO. The VCO includes a level shifter that is connected to the CCO to shift the voltage level of an oscillating signal to a supply voltage level of the level shifter.
A known technique for shifting the voltage level of an oscillating signal requires the oscillating signal and another oscillating signal from the CCO as inputs to the level shifter. This technique requires the two input signals to be exactly 180° out of phase with each other for the level shifter to maintain the duty cycle of the level-shifted oscillating signal equal to that of the input oscillating signals. However, since the CCO contains an odd number of cascade-connected inverters, the phase difference between any two oscillating signals is not 180°, and hence, this technique does not provide an effective solution. Further, in order to match capacitive loading at each phase of the CCO, a level shifter is required for each oscillating signal, leading to an increase in power consumption and circuit area.
Another known technique is to use a bias circuit along with the level shifter to adjust the operating points (i.e., the quiescent points) of transistors of the level shifter using the ring voltage. However, this technique requires a bias circuit for each oscillating signal, which increases power consumption and circuit area.
It would be advantageous to have a level shifter that does not increase circuit area and power consumption, and maintains the duty cycle of the level-shifted oscillating signal substantially equal to that of the oscillating signal input to the level shifter.